Screw press for dewatering a slurry

ABSTRACT

PCT No. PCT/JP91/01268 Sec. 371 Date May 17, 1993 Sec. 102(e) Date May 17, 1993 PCT Filed Sep. 24, 1991 PCT Pub. No. WO93/05953 PCT Pub. Date Apr. 1, 1993.A screw press for dehydrating slurry and comprising an outer screen casing, a screw shaft, a slurry supplying portion, and a driving unit which rotates the outer screen casing and the screw shaft. The driving unit rotates the screw shaft in one rotational direction and the outer screen casing in the opposite rotational direction. The driving unit comprises a transmission for changing the rotational frequency of at least one of the outer screen casings or the screw shaft. It is possible to have a high dehydrating effect by rotating the outer screen casing in the opposite rotational direction of the screw shaft at a predetermined rotational frequency.

FIELD OF THE INVENTION

This invention relates in general to screw presses, and relates inparticular to a screw press which dehydrates slurry to produce sludgeand discharges the sludge.

BACKGROUND OF THE INVENTION

A conventional screw press generally has a screw shaft mounted inside anouter screen casing. Slurry is supplied between the screw shaft and theouter screen casing. Slurry is then dehydrated and pressed by rotatingthe screw shaft subject the slurry to a solid-liquid separation, and theproduced sludge is discharged as a cake.

When the cake is formed gradually during the dehydrating operation bythe screw press, the load on a drive unit rotating the screw shaftbecomes too heavy to press the slurry sufficiently.

The above mentioned outer screen casing mounted on the screen press isnot capable of bearing a large pressure. This is because the outerscreen casing is mainly formed from a metal screen. The screw press fordehydrating viscous waste water requires a pressure tightness in orderto receive a large pressure. Therefore, the metal screen of the outerscreen casing mounted on the press is rigidly reinforced by rings,flanges, and so on. The screen of the screw press processing the viscousslurry usually has a fine mesh. As a result, the screen tends to clogand then needs to be cleaned. Conventionally, although the cloggedscreen is cleaned with a brush, it is very difficult to clean theclogged screen to a good condition because the screen has a very finemesh and the above mentioned reinforced flange and related structureprevent the brush contacting the entire screen. In an alternative methodof cleaning the screen by spraying compressed air onto the screen, thatmethod also is unable to thoroughly remove the clogging.

SUMMARY OF INVENTION

It is a primary object of this invention to provide a screw press whichhas an improved capability of dehydration and is capable of reducing anoverload on a drive unit rotating a screw shaft during the dehydration,and has a screen from which the clogging of the mesh can be easilycleaned.

It is another object of this invention to provide a method for drivingthe screw press, comprising a first step of detecting the overloadproduced in a drive unit driving the screw press during processing ofslurry, and a second step of returning to the slurry process again afterreducing the overload, thereby performing the slurry processcontinuously and efficiently.

This invention is based on the discovery that the slurry process iseffectively performed by rotating an outer screen casing simultaneouslywith the rotation of the screw shaft at a rotating speed within apredetermined range, in the opposite rotational direction of the screwshaft. The screw press of this invention is characterized by a driveunit for rotating the screw shaft in one rotational direction and forrotating the outer screen casing in the opposite rotational direction atthe same time. The drive unit has a transmission which changes therotational speed of at least one of the outer screen casing or the screwshaft.

The effectiveness of dehydration by the screw press is especiallyobtained by setting the rotational speed of the outer screen casing inthe ratio 0.1-1.2 to that of the screw shaft. Therefore, thetransmission is characterized by the capability of rotating the outerscreen casing and the screw shaft in accordance with the above ratio.

The above-mentioned screw shaft is characterized by a hollow shafthaving an outer screenlike surface for filtering the slurry. Therefore,the dehydration efficiency becomes higher by performing a doublefiltration.

The above-mentioned screw press comprises a device for detectingoverload when it occurs in the drive unit, and a device for rotating atleast one of the outer screen casing and the screw shaft in a rotationaldirection opposite to their present rotational direction for apredetermined period of time against said overload. Therefore, the loadof the drive unit is reduced.

In the screw press above described, a high pressure cleaning device isdisposed inside the screw shaft and on the portion adjacent to the outersurface of the outer screen casing. Therefore, it is possible to reducethe overload by cleaning the screen and the contact surfaces of theouter screen casing and the screw shaft with the cake, by using a devicewhich injects water or a wash liquid at high pressure. The cleaningdevice is also used for cleaning the outer screen casing and the screwshaft after completing the dehydration.

In the method of the present invention for driving the screw press, thedrive unit rotates at least one of the outer screen casing or the screwshaft in a rotational direction opposite to an initial rotationaldirection for a predetermined period of time. Thereafter, the drive unitreturns to the initial driving condition to rotate the outer screencasing and the screw shaft in the initial rotational direction.

When overload is produced in the drive unit during the above drivingmethod, it is possible to reduce the overload by cleaning cake from thecontacting surface of the outer screen casing and the screw shaft usingthe high pressure cleaning device.

Other objects and advantages of the present invention will becomeapparent from the following description of the preferred embodiment.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partially sectioned side view of a screw press according toan embodiment of the present invention.

FIG. 2 is a plan view of the screw press shown in FIG. 1.

FIG. 3 is a side view of the screw press taken from the right side ofFIG. 1 and shows one portion sectioned on line III--III of FIG. 1.

FIG. 4 is a side view of the screw press taken from the left side ofFIG. 1.

FIG. 5 is a cross-sectional view taken on line V--V of FIG. 1.

FIG. 6 is a perspective view showing a high pressure cleaning device forcleaning the outer screen casing and the screw shaft of the screw press,and the meshes of the outer screen casing.

FIG. 7 is a cross sectional view taken on line VII--VII of FIG. 6showing a double filter.

FIG. 8 is a cross sectional view showing the screw shaft decenteringrelative to the outer screen casing.

FIG. 9 is a diagram showing various driving units of the screw press ofFIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIGS. 1 and 5, a screw press 1 according to the disclosedembodiment of this invention is mounted on a main support 2. As shown inFIG. 1 and FIG. 5, a frame 3 is secured to the main support 2. Threerollers 4 are disposed on two portions of the frame 3, respectively. Twoof the three rollers 4 are disposed on the lower portion of the frame 3and the other roller is disposed on the center of the upper portion ofthe frame. An outer screen casing 5, which is mainly made from a metalmesh, is reinforced and integrated with a plurality of rings 6. Theouter screen casing 5 is supported horizontally by the rollers 4 througha pair of rings 7 at both ends of the outer screen casing. Asillustrated in FIGS. 2 and 5, a driven gear 8 is disposed on the outerleft end of the outer screen casing 5. On the other hand, as illustratedin FIGS. 1 and 3, the right end of the outer screen casing 5 isconnected through a flange 9 with a hopper 10 which serves as a slurrysupplying part. The hopper 10 has a rectangular cylindrical shape and isprovided with a mesh basket 11 on the inside thereof. The mesh basket 11has a lower portion having a semi-cylindrical shape at the position ofelongating a lower semi-circle of the outer screen casing 5. A chute 13is placed under the mesh basket 11.

The slurry added flocculant is supplied to the hopper 10 from the upperend thereof. Solid material produced by flocculating the slurry issupplied into the hopper without being destroyed because there is nopipe for supplying the slurry in the hopper 10. The solid material ofthe slurry is precipitated and the supernatant liquid thereof stays inthe tipper part of the hopper 10. The supernatant liquid is lead throughtwo drains 14 to the chute 13 mounted under the hopper 10, and thendrained from a drain dish 15 which is disposed below the hopper andsupported by the main support 2. The slurry at the bottom of the hopper10 is filtered through a mesh 12 on the lower portion of the mesh basket11. The filtrate is then drained to the drain dish 15 through the chute13. Consequently, the solid material is mainly left on the bottom of thehopper 10 and the slurry-supplying part serves as a thickener.

A circular cone 18 is arranged coaxially inside the outer screen casing5. A base end which is a taper portion of the circular cone 18 ispositioned at the bottom portion of the hopper 10 and is protrudedtherefrom. The diameter of the circular cone 18 becomes larger towardthe opposite end, so that the space between an outer surface of thecircular cone 18 and the outer screen casing 5 becomes graduallynarrower. Both ends of the circular cone 18 are rotatably supported bybearings 21 which are secured to the frame 3. A spiral wing 22 extendsall along the length of the outer surface of the circular cone 18 toform a screw shaft 20.

A motor 25 (FIG. 2) is mounted on the main support 2 parallel with theouter screen casing 5. A driving shaft 27 of the motor 25 is providedwith a transmission 26 comprising a plurality of pinions for engaging adriven gear 8. When rotating the driving axis 27 clockwise by drivingthe motor in FIG. 5, the pinion 28a (or 28b) of the transmission 26rotates likewise. The pinion 28a or 28b is selected to engage with thedriven gear 8 of the outer screen casing 5. As a result, the outerscreen casing 5 rotates counterclockwise. Other pinions (not shown) thanpinions 28a, 28b can also be selected and thereby the rotational speedof the outer screen casing 5 can be varied.

Because the pinion 28a or 28b rotates downwardly where that pinionengages with the driven gear 8, a downward force is produced to pressthe outer screen casing 5 downwardly. The two lower rollers 4 make theouter screen casing 5 stable against the above mentioned force, namely,those two rollers support the outer screen casing 5 steadily withoutdecentering the screw shaft 20. The driving shaft 27 of the motor 25further extends through the gear box 26 and is pivoted by a plurality ofbearings 28 secured to the main support 2. A sprocket wheel 29 ismounted on the top :of the driving shaft 27.

A shaft 30 is arranged parallel to the driving axis 27 of the motor 25and is supported rotatably by the other bearing 31 secured to the mainsupport 2. A sprocket wheel 32 is secured to one end of the shaft 30 andthe other end is rigidly secured to the screw shaft 20. The sprocketwheel 29 is secured to the driving axis of the motor 25 and the sprocketwheel 32 is secured to the shaft 30. A chain 33 extends around thesprocket wheel 29 and the sprocket wheel 32 to transfer the rotation ofthe motor 25 to the screw shaft 20. The screw shaft 20 rotatesclockwise, that is, in the opposite rotational direction to therotational direction of the outer screen casing 5. The motor 25 iscontrolled by a control board 35.

As illustrated in detail in FIGS. 6 and 7, the circular cone 18 is ahollow circular cone casing. The circular cone casing is in the form ofa screen the same as the outer screen casing 5. As the spiral wing 22extends to the bottom portion of the hopper 10, when the screw shaft 20rotates, a slurry S moves immediately along the spiral wing 22 and iscarried to the left side of the spiral wing. At the same time, theslurry S is pressed between the outer screen casing 5 and the circularcone 18, and the slurry is filtered by double filters formed by theouter screen casing 5 and the circular cone. A filtrate F drainedoutside the outer screen casing 5 drains down to the drain groove 15 tobe drained. The filtrate F drained inside the circular cone 18 isdrained through a drain 39.

The screens of the outer screen casing 5 and the circular cone 18gradually become finer from the hopper 10 toward a drain exit 40 of acake C. This is because the moisture content of the sludge becomes lowerfrom the hopper toward the drain exit 40 of the cake C. An example ofthe screen of the outer screen casing 5 will be described as follows.The size of the mesh of the screen is set for three grades M1, M2 and M3from the hopper side as shown in FIG. 6. M1 is a 2 mm-mesh screen with anumerical aperture of 40%. M2 is a 1 mm-mesh screen with a numericalaperture of 22.5%. M3 is a 0.5 mm-mesh screen with a numerical apertureof 18.6%.

Furthermore, if the size of the mesh of the screen in the circular cone18 is smaller than that of the outer screen casing 5, it would bepossible to have a superior water break to sludge including rich-fiberand to increase the quantity of sludge to be treated.

Cleaning pipes 41 and 42, which inject high pressure water, are disposedon the outer portion of the outer screen casing 5 and inside the screwshaft 20, respectively. These cleaning pipes 41 and 42 are connected toa water tank as described below. The high pressure water is force fed tothe cleaning pipe 41 and 42 by a pump which is controlled by the controlboard 35.

The motor 25 serves as a drive unit which rotates the outer screencasing 5 and the screw shaft 20. The motor 25 can be overloaded when acake is formed as the sludge comes to have high density content duringprocessing the slurry, or when the screen is clogged. It is preferred todispose a detector for detecting the overload as described below. As theoverload is detected, it is possible to reduce the load by operating thecontrol board 35 to make the motor 25 rotate backward to rotate theouter screen casing 5 and the screw shaft 20 in the opposite rotationaldirection to the initial rotational direction, respectively. The abovementioned backward rotation is to be performed for a predeterminedperiod of time. By injecting high pressure water from the cleaning pipe41 and 42, during the above mentioned time of the backward rotation, itis possible to clean all the screens of the outer screen casing 5, thescrew shaft 20, all the contacting surface of the cake formed on thescreen, and to further reduce the load of the drive unit 25.

The description now describes the effect of oppositely rotating theouter screen casing 5 relative to the screw shaft 20. Charts 1 and 3attached to the end of the description indicate the results of theexperiments of dehydrate processing the various kinds of slurry by usingthe screw press of the present invention (the screw press proved to becapable of also inhibiting outer screen casing 5 from being rotated).

Chart 1 shows a result of the experiment of dehydrate-processing slurryproduced by flocculating a paper drainage. This experiment was performedby backwardly rotating the outer screen casing 5 and the screw shaft 20by changing both rotational speeds N1 and N2 to equalize the differenceN1-N2 (the sum of absolute value of their rotational speed) of bothrotational speeds.

Chart 2 shows a result of the experiment of dehydrate-processing sludgeproduced by a sewerage disposal plant, This experiment was performed forone case that the outer screen casing 5 was fixed (the outer screencasing 5 having a rotational speed N2=0) and the screw shaft 20 wasgradually revved up, and for the other case that rotational speed N2(the backward rotation) of the outer screen casing 5 was graduallyrevved up relative to the screw shaft 20.

Chart 3 shows a result of the experiment of dehydrate-processing slurrywhich is produced by flocculating and depositing a paper drainage. Thisexperiment was performed by gradually revving up (backward rotation) theouter screen casing 5 relative to the rotation of the screw shaft 20.

According to Chart 1, Test No. 1 was performed with the screw shaft 20having a rotational speed N1 of 0.6 rpm. The outer screen casing 5 has arotational speed N2 of -0.3 rpm, so as to have the difference of therotational speeds N1-N2 of 0.9 rpm. Test No. 2 was performed with thescrew shaft 20 having rotational speeds N1 of 0.9 rpm, the outer screencasing 5 having rotational speeds N2 of 0, that is, the outer screencasing 5 was fixed to set the difference of rotational speeds to be also0.9 rpm. Although the differences of rotational speeds are the same 0.9rpm, Test No. 1, by backwardly rotating the outer screen casing 5,resulted in 56.4% of the moisture content and 35.6 Kg-DS/hr ofprocessing amount of the dry cake, and had higher processing effect incomparison with Test No. 2 by fixing the outer screen casing 5,resulting in 57.9% of the moisture content and 33.3 Kg-DS/hr of theprocessing amount of the dry cake. The same results could be obtained inTest No. 3 and No. 4, and No. 5 and No. 6.

In the test shown in Chart 2, when the outer screen casing 5 was fixedand the rotation of the screw shaft 20 was revved up, the moisturecontent and the processing amount became larger (Tests No. 7-9). On theother hand, when the rotational speed N1 of the screw shaft 20 wasunchanged and the rotational speed N2 of the outer screen casing 5 wasgradually revved up, the moisture content was almost constant but theprocessing amount was increased a great deal (Tests No. 10 and 11, No.12 and 13, No. 14-16). However, when the rotational speed of the outerscreen casing 5 becomes more than a certain degree relative to the screwshaft 20, an increasing rate of the moisture content became larger incomparison with that of the processing amount (Tests No. 15 and 16).

In the test shown in Chart 3, when the rotational speed (backwardrotation) of the outer screen casing 5 revved up with fixed rotationalspeed of the screw shaft 20, the moisture content was almost unchangedbut the processing amount increased (Tests No. 19 or 22). However, whenthe rotational ratio N2/N1 of the outer screen casing 5 to the screwshaft 20 was increased more than a certain degree, the moisture contentbecames larger (Tests No. 18 and 23).

Therefore, it is obvious that the dehydrating effect is increased byrotating the outer screen casing 5 in the opposite rotational directionto the rotational direction of the screw shaft 20. Furthermore, therotational ratio N2/N1 of the rotational speed N2 of the outer screencasing 5 to the rotational speed N1 of the screw shaft 20 is preferablyabout 0.1 at the minimum and 0.8˜1.2 at the maximum. It will beunderstood that the driving force to the slurry is produced by thespiral wing 22 and friction force is produced between the slurry and aninner surface of a slurry chamber defined by the outer screen casing 5and the screw shaft 20, and the driving force and the friction forcemultiply act on the slurry during backward rotation of the outer screencasing 5 at a low speed relative to the screw shaft 20 to rapidly movethe slurry and to effectively dehydrate the slurry. It will be alsounderstood, when further revving up the rotation of the outer screencasing 5, that the slurry slips on the inner surface of the slurrychamber to suppress the dehydrating effect and to increase the moisturecontent.

As to the other effect by backward rotation of the outer screen casing 5against the screw shaft 20, it is possible to drain the cake having auniform thickness and moisture content from the drain exit 40 even ifthe screw shaft 20 and the outer screen casing 5 are decentered or thespiral wing is partially abraded. FIG. 8 is a explanatory drawing of theeffect, and shows the condition of the screw shaft 20 decenteredrelative to the outer screen casing 5. As long as the outer screencasing 5 is fixed, it is impossible to unify the cake since decenteringpoints C1 and C2 are always placed on the same positions. However, ifthe outer screen casing 5 rotates backwardly, it is possible to unifythe cake because of changing the positions of the decentering points C1and C2.

FIG. 9 shows various drive units each of which drives the abovementioned screw press. At the screw press of the above mentionedembodiment, the screw shaft 20 and the outer screen casing 5 arerotatably driven by the motor 25. A first transmission 25 is mountedonly on a driving series of the outer screen casing 5 but not on aseries of screw shaft 20. The diagram of the FIG. 9 shows a modifiedexample of screw press having a second transmission 46 for shifting agear on the driving series of the screw shaft 20 to be able to suitablychange the rotational speed of the screw shaft 20. A load detector 48for detecting the load is disposed on the motor 25.

The description will be made with regard to a method of driving thescrew press 1 with reference to the Figures.

At first, the first and second transmissions are set for rotating thescrew shaft 20 and the outer screen casing 5 at an appropriaterotational ratio, Then the motor 25 is driven by operating the controlboard 35 to rotates the screw shaft 20 in one direction and the outerscreen casing 5 in the opposite direction. The screw shaft 20 is usuallyrotated at the speed of 1-10 rpm. Therefore, the slurry in the slurrysupplying part (not shown) is transferred along the spiral wing 22 to bedehydrated and pressed. The formed cake is discharged from the drainexit 40. A ring 55 having a taper surface is disposed in the drain exit40. The ring 55 is connected to a piston rod 54 having two hydrauliccylinders. The cylinders 53 are driven to operate as the control board35 drives the oil pressure pump unit 52. Therefore, it is possible toset the position of the ring 55 by moving the ring right or left. It ispossible to adjust the amount of draining of the cake and the amount ofthe pressure force pressing the cake by controlling the position of thering 55.

When the pressed cake has a high viscosity or solidity and when thescreens of the outer screen casing 5 and the screw shaft are clogged,the motor 25 suffers from overload and then the screw press does notwork sufficiently. When the load of the motor 25 approaches apredetermined degree, the load detector 48 detects that load andtransmits a signal to the control board 35. Upon receiving that signal,the control board 35 is operated manually or automatically to rotate themotor 25 backwardly for a period of time. Therefore, the screw shaft 20and the outer screen casing 5 rotate in the opposite rotationaldirections to the present rotational directions, respectively, to reducethe load of the motor 25. When the motor 25 is operated to be rotatedbackwardly, the control board 35 automatically actuates the pump 50 forthe above mentioned period of time to feed the water inside the watertank 49, connected to the pump, into the cleaning pipes 41 and 42 athigh pressure. Accordingly, the high pressure water is injected from thecleaning pipes 41, 42 to clean the inner and outer surfaces of the outerscreen casing 5 and the screw shaft 20 and the contact surface thereof.In other words, the screens of the outer screen casing 5 and the screwshaft, the connecting surfaces of the outer screen casing 5, the screwshaft 20 and the cake am cleaned to further reduce the rotationalresistance on the contact surface, further reducing the load of thedriving motor 25.

The present invention should not be limited to the above mentionedembodiments, and preferably should be capable of being modified. Forinstance, it is possible to mount the pinion 28a and the driven gear 8of the outer screen casing 5 thereon without the transmission for thedrive unit, and to set these gear ratios to the predetermined value, andthe ratio of rotation of the screw shaft 20 and the outer screen casing5 to a predetermined value.

Although, in the above embodiment, the outer screen casing 5 and thescrew shaft 20 are driven by one drive unit 25, it is possible todispose two drive units and drive the outer screen casing 5 and thescrew shaft 20, respectively. It is further possible to dispose thetransmission on one or both drive units to separately set the rotationalspeed of the outer screen casing 5 and the screw shaft 20, respectively.

It is also possible to dispose one drive unit as indicated in the aboveembodiment, and to dispose one transmission next to the drive unitwherein the transmission is capable of changing the rotational speed ofeither the outer screen casing 5 or the screw shaft 20 or both.

As an alternative to the transmission operated by a gear shift,transmissions operated by a pulley, sprocket wheel, or other knowntransmissions may be used.

In the above embodiment, the outer screen casing 5 is in shape of acylinder and the screw shaft 20 is in the shape of a circular cone. Asan alternative to the above, the outer screen casing 5 can be in theshape of a circular cone, and the screw shaft 20 can be in the shape ofa cylinder or in other shapes as long as a relative space between themnarrows in the direction of extending the screw shaft 20.

In the above embodiment, there are three grades in the size of the meshof the screen and the numerical aperture, but the grades alternately maybe two, four or more. It is also possible to set the size of the meshand the numerical aperture gradually smaller in the direction of thescrew shaft, without any steps.

The screw press of this invention, as described above, has an excellentcapability of processing dehydration. Moreover, the screw press iscapable of resolving an overload to continue dehydration when the pressdoes not work sufficiently as a consequence of producing the overload.Furthermore, it is possible to utilize the screw press of this inventionin other industries because the screen press of this invention canprocess various kinds of slurries.

                  CHART 1                                                         ______________________________________                                              Rotational                                                                              Rotational                                                          Speed of  Speed of  Differ-                                                                             Moisture                                            the Screw the Outer ence  Content                                                                              Amount of                              TEST  Shaft N1  Casing    N2 -  of Cake                                                                              Processing                             No.   (rpm)     N2 (rpm)  N1    (%)    Dry Cake                               ______________________________________                                        1     0.60      -0.30     0.90  56.4   35.6                                   2     0.90      0         0.90  57.9   33.3                                   3     0.90      -0.45     1.35  56.6   38.0                                   4     1.35      0         1.35  60.1   37.3                                   5     1.20      -0.60     1.80  60.2   54.4                                   6     1.80      0         1.80  61.8   50.4                                   ______________________________________                                    

                  CHART 2                                                         ______________________________________                                              Rotational                                                                              Rotational                                                          Speed of  Speed of        Moisture                                            the Screw the Outer       Content                                                                              Amount of                              TEST  Shaft N1  Casing    Ratio of Cake                                                                              Processing                             No.   (rpm)     N2 (rpm)  N2/N1 (%)    Dry Cake                               ______________________________________                                         7    0.380     0         0     82.1   4.7                                     8    0.446     0         0     82.5   5.3                                     9    0.558     0         0     83.1   7.8                                    10    0.255     0.101     0.40  82.0   4.5                                    11    0.255     0.202     0.79  81.0   5.7                                    12    0.380     0.085     0.22  82.3   5.7                                    13    0.350     0.174     0.46  81.3   7.4                                    14    0.446     0.085     0.19  81.8   7.2                                    15    0.446     0.223     0.50  81.8   9.1                                    16    0.446     0.347     0.78  83.0   9.4                                    ______________________________________                                    

                  CHART 3                                                         ______________________________________                                              Rotational                                                                              Rotational                                                          Speed of  Speed of        Moisture                                            the Screw the Outer       Content                                                                              Amount of                              TEST  Shaft N1  Casing    Ratio of Cake                                                                              Processing                             No.   (rpm)     N2 (rpm)  N2/N1 (%)    Dry Cake                               ______________________________________                                        17    0.558     0.438     0.78  53.9   27.7                                   18    0.558     0.893     1.60  60.7   24.8                                   19    1.010     0         0     51.9   22.6                                   20    1.010     0.202     0.20  52.4   26.2                                   21    1.010     0.438     0.43  54.5   28.2                                   22    1.010     0.695     0.69  55.6   30.2                                   23    1.010     0.893     0.89  63.1   29.2                                   ______________________________________                                    

We claim:
 1. A screw press (1) comprising:a main support (2); a frame(3) secured to said main support (2); an outer screen casing (5)rotatably supported by said frame (3) to extend horizontally above saidmain support (2), said outer screen casing having a slurry supplyingside at a first end and a cake discharging side at a second end, saidouter screen casing (5) having mesh screens (M1, M2, M3) with a finermesh on said cake discharging side (40, M3) compared with a mesh on saidslurry supplying side (10, M1); a screw shaft (20) rotatably mounted onsaid main support (2) coaxially inside said outer screen casing (5) toextend horizontally above said main support (2) so as to graduallyreduce a space between said screw shaft (20) and said outer screencasing (5) in the extending direction, said screw shaft (20) beingprovided with a spiral wing (22) arranged around an outer surface ofsaid screw shaft (20) along its extending length so as to substantiallycontact with said outer screen casing (5), and said screw shaft (20)having an outer surface with a mesh screen of a finer size compared withsaid mesh screens of said outer casing (5); a slurry supplying means(10, 11, 14) disposed at said first end of said outer screen casing (5)to supply a slurry into said space between said outer screen casing (5)and said screw shaft (20); and at least one rotating means (25) forrotating said screw shaft (20) in one rotational direction and saidouter screen casing (5) in the opposite rotational direction.
 2. A screwpress as claimed in claim 1, wherein said one rotating means (25)simultaneously rotates said outer screen casing (5) and said screw shaft(20) in opposite directions from each other.
 3. A screw press as claimedin claim 1, wherein said one rotating means (25) is operative to rotatesaid outer screen casing (5) at a ratio of between 0.1 to 1.2 therotating speed of said screw shaft (20).
 4. A screw press as claimed inclaim 3, wherein said rotating means (25) includes a transmission (26,46) for changing the rotational frequency of at least one of said outerscreen casing (5) and said screw shaft (20).
 5. A screw press as claimedin claim 1, further comprising a detector (48) for detecting a load onsaid rotating means (25), and further comprising a backward rotatingunit (25, 35) responsible to said load detector for rotating at leastone of said outer screen casing (5) and said screw shaft (20) indirections opposite from said directions produced by said rotatingmeans, when said detected load is substantially a predetermined amountindicative of an overload on said rotating means.
 6. A screw pressclaimed in claim 1, wherein said screw shaft (20) has a hollow shape andsaid outer surface is in the form of a mesh screen for discharging aseparated supernatant liquid produced by expressing slurry.
 7. A screwpress as claimed in claim 1, further comprising high pressure cleaningdevices (41, 42) disposed outside said outer screen casing (5) forcleaning said outer screen casing and inside said screw shaft (20) forcleaning said screw shaft.
 8. A screw press as claimed in claims 1 or 7wherein said slurry supplying means (10, 11, 14) includes means (11, 12,14) for draining a separated supernatant liquid of said slurry suppliedto said slurry supplying means to concentrate said slurry.
 9. A screwpress as claimed in claim 1, wherein said slurry supplying means (10,11, 14) includes a vertical hopper (10), said hopper (10) having abottom portion communicating with said first end of said outer screencasing (5), and said screw shaft (20) having an end portion extendingbelow the bottom portion of the hopper.